Solidify modifier thinner near edges
I'm not sure what is exactly your problem as your mesh is a bit messy.

First, to clear you object a bit: In Object mode, ctrl A to apply scale. In Edit mode, select all and W > Remove Doubles, and also ctrl N to recalculate the normals.

Now, you have inner faces, it makes the Solidify a bit complicated because it will not only thicken the external walls but also the internal, and you will probably have faces crossing and overlapping each others. So if you want to use Solidify you should better create separate objects, ie separate floors form walls from roof.

I hope it will fix your problems...